The present invention concerns a process for the selective reduction of water inflows from an oil or gas-producing formation towards a producing well without negative effect on the oil or gas production. This process is based on the injection into the formation, from said producing well, of an aqueous dispersion or solution of a nonhydrolyzed polyacrylamide or of a nonhydrolyzed acrylamide copolymer, followed with an alkali metal basic compound, for example a hydroxide, carbonate, phosphate, alcoholate or silicate, for the hydrolysis in situ of said polyacrylamide or copolymer.
Water, as brine, is often present in oil or gas-containing formations. The process of bringing into production of oil- or gas-containing reservoirs results in the concomitant flow into the producing well of water in such amount that great difficulties quickly arise. Such difficulties include formation of deposits in the well vicinity or in the tubing; increased corrosion of bottom or surface metal parts; increase of the pumped, transferred or stored fluid amounts; formation with oil of emulsions, which are sometimes difficult to break at the surface; and termination of the production of the wells.
Many methods for reducing water inflows in producing wells have been proposed and tested on the field. They generally consist of placing into the formation, at the water-oil or- gas interface, an impermeable barrier consisting of, for example, cements, resins or suspensions of solid particles. These plugging agents have the disadvantage of blocking oil or gas almost as mush as water, particularly when the water inflow results from the so-called coning phenomenon (formation of a water cone).
More recently, it has been proposed to use hydrosoluble polymers, particularly hydrolyzed polyacrylamides and various polysaccharides, optionally cross-linked by salts of multivalent, e.g trivalent, ions such as chromium and aluminum ions. It has been observed that, whereas this reticulation in the form of more or less compact gels results effectively in the discontinuation or reduction of the water inflow, it suffers from the disadvantage of also reducing to a large extent the oil or gas production.
Among the hydrosoluble polymers, polyacrylamides, particularly hydrolyzed polyacrylamides, are currently used to prevent water inflows in the producing wells. Their mechanism of operation is understood to be the following: the polymer, injected into the porous medium as aqueous solution, is adsorbed at the solid surface and reduces the pore diameter of the porous medium by swelling in contact with water; accordingly, the polymers slow down the water flow. On the contrary, non-aqueous fluids such as oil or gas do not swell the adsorbed macromolecules which, consequently, allow free passage to the flow of these fluids.
U.S. Pat. No. 3,308,885 discloses the use of hydrolyzed polyacrylamide in the production water. The method is not completely satisfactory, in particular in a formation of high permeability to water. As a matter of fact, such formations require the use of high polymer concentrations, resulting in difficulties in injecting the obtained highly viscous solutions. On the other hand, in this type of porous medium, the content of clay, hence of adsorbing sites, being relatively low, the polymer adsorption becomes more difficult.
U.S. Pat. No. 4,095,651 copes with some of these defects by recommending the dispersion of the hydrolyzed polyacrylamide in a water of higher salt content than that of the production water. The ionic strength increase of the polymer dissolving water has the effect of reducing the viscosity of the injected solutions, hence the pumping difficulties, and provides for a higher adsorption of the hydrolyzed polyacrylamide onto the wall of the porous media. However, the use of an injection water of higher salt content than that of the production water requires the addition of large salt amounts and often results in fluid-fluid and fluid-rock incompatibilities.